Orthogonal Frequency-Division Multiplexing (OFDM)
OFDM uses multiple orthogonal subcarriers to transmit information at a relatively low symbol rate. As an advantage, OFDM can withstand severe changes in channel state and quality, such as high frequency attenuation, narrowband interference, and frequency-selective fading due to multipath, using a single carrier. Channel equalization is simplified because OFDM uses slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. A low symbol rate enables guard intervals and time-spreading, while reducing inter-symbol interference (ISI). Some of the subcarriers in some of the OFDM symbols carry sounding signals for estimating the channel state, and for performing synchronization.
Orthogonal Frequency Division Multiple Access (OFDMA)
As a disadvantage, OFDM does not provide multi-user channel access to a channel. OFDMA provides this capability by time, frequency or coding separation of multiple users. That is, frequency-division multiple access is achieved by assigning different OFDM subchannels to different users (mobile stations). A subchannel is a group of subcarriers, which need not be contiguous. OFDMA is used in the uplink of networks according to the IEEE 802.16 Wireless MAN standard, commonly referred to as WiMAX. Some common terms used in the standards and herein are defined in the Appendix
Sounding
In order to allocate the frequencies of the subchannels and subcarriers most efficiently, the mobile station can transmit sounding signals. The base station can obtain channels state information (CSI) from the sounding signals, and allocate the frequencies accordingly.
The IEEE 802.16 standards define sounding signals for the uplink (UL) from mobile stations to the base station. Two types of carrier allocations are defined: non-distributed subcarrier allocation (Type A), and distributed subcarrier allocation (Type B).
Type A, which is mandatory in the current WiMAX forum profile, provides for mobile station multiplexing, decimated separability, and cyclic shift separability. Multiple antennas are only supported when the number of antennas is equal to the number of RF chains.
Type B, which is optional in the WiMAX forum profile does not allow mobile station multiplexing and does not support multiple antennas.
Antenna Selection
Networks according to the IEEE 802.16 standards support multiple antennas and radio frequency (RF) chains in base stations (BSs) and mobile stations (MSs). Due to the high cost of RF chains and relatively low cost of antennas, the number of RF chain (N) can be less than the number of antennas (M), that is, N≦M. However, as stated above, both Type A and B do not support antenna selection.
Therefore, it is desired to select antennas in OFDAM networks. However, the current OFDAM standards, as described above, do not support sounding for the purpose of antenna selection. Therefore, it is desired to adapt the current sounding signals for the purpose of antenna selection.